A fluorine-containing resin is excellent in all of heat resistance, chemical resistance, electrically insulating properties and high frequency characteristics, and also has non-tackifying property and low abrasion property which are inherent to the fluorine-containing resin, and therefore is used in wide fields, such as space and aeronautic industries and home products as well as chemical industries, electrical and electronic industries and machine industries. Among the fluorine-containing resins, polytetrafluoroethylene (PTFE) and tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer (PFA) are particularly excellent in heat resistance and chemical resistance, and therefore, are preferably used as materials essential to wafer carriers, bottles, washing tanks, filters, piping systems such as pumps, tubes and couplings, or linings for chemical tanks and valves in the semiconductor manufacturing field. Particularly in the wet process which occupies 20% of the whole semiconductor manufacturing process, since highly reactive and corrosive chemicals such as strong acids and strong alkalis are used at a very high temperature, resins for general uses such as polyethylene and poly(vinyl chloride) and metallic materials such as stainless steel cannot be used in such use. At present, only fluorine-containing resins such as PTFE and PFA are usable. However with the advancement of highly integrated highly dense LSI in recent years, it was found out that even fluorine-containing resins being excellent in the above-mentioned properties have some problems. In a process of manufacturing VLSI, since contaminations such as presence of a very small amount of particles, metals and organic contaminants remarkably affect its yield, an ultra-clean manufacturing process is required. Accordingly those fluorine-containing resin molded articles which often come to contact with a silicon wafer through chemical solution naturally require ultra cleanness. However the surface of the fluorine-containing resin molded articles which have been hitherto available is markedly rough, and thus particles, metal impurities, organic impurities, low molecular weight components (oligomer) of those resins and the like adhere to the above-mentioned uneven surface. These contaminants are difficult to remove by washing with ultra pure water and highly pure chemicals, and gradually flow out into the ultra pure water and highly pure chemicals during a long period of time in the wet process, and thereby silicon wafers are contaminated. Also those impurities removed from the silicon wafers through wet cleaning adhere again to the uneven surface of the fluorine-containing resin, and undesirably flow out again into the ultra pure water and highly pure chemicals. That is to say, if the surface of the fluorine-containing resin is uneven, adhesion and removal of those finely-divided impurities to and from the resin surface is repeated, and therefore, there is a risk such that silicon wafers not to mention of the ultra pure water and highly pure chemicals are contaminated.
Since PTFE has a very high melt viscosity (10.sup.11 to 10.sup.13 poises at 380.degree. C.), its molding cannot be conducted by a normal plastic molding method such as melt extrusion molding and injection molding, but there is used a specific method such that after raw material powder is previously compressed by a proper force, the pre-molded article is heated to a temperature of not less than its melting point and then sintered. Thus the molding is carried out by the compression molding, ram extrusion molding, paste extrusion molding, calender molding and the like, but in these molding methods, since the resin is not made uniform by melting, voids are easy to be generated in the molded article, and the surface of the molded article becomes uneven and rough. Therefore, the surface roughness of the PTFE molded article is considerably large as compared with that of other thermoplastic resins. The surface roughness (Ra) comes to be not less than 2,000 .ANG..
On the contrary, a melt viscosity of PFA is as low as 10.sup.4 to 10.sup.6 poises at 380.degree. C., and a normal plastic molding such as injection molding, extrusion molding, blow molding or compression molding is used. Since the resin is made uniform by melting, the surface of the molded article is smoother than that of PTFE, but a spherulite structure having a diameter of about 20 .mu.m to about 50 .mu.m tends to remain and the surface roughness (Ra) is not less than 700 .ANG..
As mentioned above, a detailed analysis indicates that the surface of the conventional fluorine-containing resin molded article is rough and not satisfactory for use with semiconductors. In order to reduce the surface roughness of the molded article, studies have been made as to smoothing of the inner surface of a mold, improvement of the raw material resin itself and the like. However the surface having the roughness Ra of not more than 700 .ANG. has not yet been obtained. Also in the field of ceramics, as a method to obtain high density and uniformity, a method of molding at a temperature as high as not less than 1,000.degree. C. and under a pressure as high as not less than 600 kg/cm.sup.2 (so-called HIP method) is known. However in such severe molding conditions, fluorine-containing resins are decomposed and deteriorated, and thus cannot be molded under such conditions.
An object of the present invention is to provide a method of smoothing a fluorine-containing resin molded article, which can assure its surface roughness (Ra) to be not more than 500 .ANG.in order to comply with the use for semiconductor manufacturing process, and to provide a fluorine-containing resin molded article having a surface roughness of not more than 500 .ANG..